MPGs are dropping .. a mystery - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #1 of 14 (permalink) Old 07-16-2017, 10:48 AM Thread Starter
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MPGs are dropping .. a mystery

Maybe someone else has run into this situation, so I thought I would post.
For many months now, the gas mileage in my ’06 6S has been sliding. There doesn't seem to be any explanation ..

 The car starts and idles and accelerates just fine
 There are no engine codes (DTCs)
 I replaced all coils and spark plugs earlier this year
 The MAF and IAT were just cleaned
 The PCV was replaced
 The filter is good
 No air leaks found
 The car coasts really well, so does not seem like brakes are dragging
 Tire pressures are good

The way it keeps dropping, it seems like something is failing, like maybe cats getting plugged or a sensor burning out. So far though, there is nothing that shows up bad.

Anyone else had a steady drop in MPGs?
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post #2 of 14 (permalink) Old 07-17-2017, 12:00 AM
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Get the cats checked Pronto. How's the oil consumption?

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post #3 of 14 (permalink) Old 07-17-2017, 02:05 AM
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How many miles on it??

If 130,000 or more "could" be the cats.
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post #4 of 14 (permalink) Old 07-17-2017, 10:52 PM Thread Starter
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Quote:
Originally Posted by GerryB View Post
Get the cats checked Pronto. How's the oil consumption?
Hmmm, that got me thinking, GerryB.

Its needed about a quart per year lately, so that did not seem like really high usage, but then again I've only been putting maybe 3,000 miles on it per year, so actually this does seem like fairly high loss. And its not leaking out to any significance.
For most of its 80,000 mile "life" [I've had it since new], the engine didn't lose even a qt between changes. Perhaps it is significant that the increase in oil usage started around the time that MPGs were dropping.

There is a small leak in the exhaust which also started about the same time as the slide in MPGs. I had wondered if maybe a failing cat had forced a leak from increased back pressure.
Another possible symptom was some oil residue in the intake when I had that apart.

How are the cats checked out? I was looking at pre and post O-2 sensor data on the Pre-cats, and that seemed about right. Do they continue to give good readings even while getting plugged up?
Is it possible to visually check out the cats condition, or is there some sort of flow measurement that is done?
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post #5 of 14 (permalink) Old 07-17-2017, 11:22 PM
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Usually the cats are tested for back pressure, but they can be checked visually. It's just not fun, especially with the V6. A LOT of work. If the cat is well and truly gone, you'll get codes for the upstream o2 sensors (the dreaded P0421).

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post #6 of 14 (permalink) Old 07-25-2017, 11:55 PM Thread Starter
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I'm getting the impression that this mystery of sliding gas mileage is going to go on for a while now.

Latest things were to do some checking of the cats. With a scan tool I checked intake vacuum when blipping the throttle and also at a constant 2,500 RPM. Both of these tests suggested that back pressure is fine.
Then I looked closer at O-2 sensor voltage while driving**. Both up and downstream signals acted normally. So I'm getting to think the cats are okay after all. And apparently the sensors are good as well.

The exhaust leak turned out to be a flanged connection at the back of the main cat. Looks like one of the bolts backed out?? This seemed odd because the bolt seemed stuck. Maybe the cat over-heated at one point and warped the flange? Or, maybe a new gasket and bolts can be installed and then be good ... for the leak anyway.

** Side note: with the scanner feeding my cell phone via blue-tooth, this was like having added gauges in the car. Pretty cool, and so nice when trouble-shooting.

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post #7 of 14 (permalink) Old 07-26-2017, 09:11 AM
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When I did get the dreaded P0421 code with my car I replaced my precats with headers. The reason I say this is because I was surprised as to how badly the substrate was breaking down inside the pre-cats yet no other warning signs.

If you really love your car, you'll go through the hell to inspect those converters visually. I also use Torque and my cell phone, but I would not trust those readings as a confirmation of my cats condition.

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post #8 of 14 (permalink) Old 07-29-2017, 01:15 PM
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Quote:
Originally Posted by GerryB View Post
Usually the cats are tested for back pressure, but they can be checked visually. It's just not fun, especially with the V6. A LOT of work. If the cat is well and truly gone, you'll get codes for the upstream o2 sensors (the dreaded P0421).

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Is there any chance you could run a long borescope up the exhaust to take a look at them?
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post #9 of 14 (permalink) Old 07-29-2017, 01:44 PM
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That would be one long boroscope. Plus you'd have to navigate the muffler and resonator, sooo...

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post #10 of 14 (permalink) Old 08-05-2017, 06:53 PM Thread Starter
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Quote:
Originally Posted by thzpcs View Post
Is there any chance you could run a long borescope up the exhaust to take a look at them?

Some DIY testing I have found involved checking back pressure by looking at engine vacuum :

A normal gauge reading should be between 16 and 21 inches of vacuum.

Have a helper snap the throttle (push down quickly and then release). There should be a brief drop in vacuum as the throttle is opened but it should return to nearly the baseline pressure you had at idle within a few seconds. If this reading takes longer than a few seconds the exhaust system is plugged.

An additional test is to keep the engine at about 2,500 RPM and watch the vacuum gauge. If the reading is low or continues to drop this is a good indication there is blockage in the exhaust system causing backpressure to build. The most likely problem is a bad converter and replacement is required.

I used an OBD II scanner and free app to do these checks of vacuum. This also worked to check voltages on the oxygen sensors above and below the pre-cats ... but as b1lk1 suggests, it is maybe a gamble to trust the engine’s health to these relatively basic tools.


b1lk1,
if you don't mind my asking, how was the gas mileage on yours when the cat(s) were going out?
Did the main cat need replacement or was it just the pre-cats?

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